U.S. Pat. No. 4,220,542, U.S. Pat. No. 4,285,814, and U.S. Pat. No. 4,390,425 disclose filtration devices used for filtering liquids which can contain various types of contamination, such as sand, dirt or coagulates. Removable filters are used, which are replaced when their permeability falls below a given limit. A specific application is found in the production of paints, where generally several filtration steps are included in several stages of production.
Filtration devices need to be cleaned regularly, for example when after filtering one batch of liquid, a new batch of another type, such as a paint of a different colour, needs to be filtered. Cleaning filtration devices is a time-consuming and laborious job. The filtered liquid tends to stick in places which are hard to reach by mere rinsing with a cleaning liquid. Generally, large amounts of cleaning liquid need to be used for rinsing. Furthermore, relatively large amounts of liquid to be filtered are still left in the device, e.g. in the inlet, when it is opened for cleaning.
Due to the size of the inlet in comparison to the volume of the filter bags in the prior art systems, the liquid to be filtered is distributed unequally over the filter bags. As a result, the filter will lose permeability at its bottom side first. When the filter is removed, an amount of liquid will still be present in the filter bag, resulting in considerable losses. U.S. Pat. No. 4,220,542 proposes to use a hollow closed container to reduce these losses. However, the container fills only a limited part of the filter bag and is hard to clean. Furthermore, unequal flow distribution also reduces the efficiency of rinsing with a cleaning liquid.